In a cellular communication system, a geographic area is divided into multiple cells 10, 12, 14 as shown in FIG. 1. At the center of each cell 10, a base station 22 is located to serve mobile stations 28, 30 in the cell. Having small, multiple cells help increase coverage and capacity. Each cell can be further divided into sectors 16, 18, 20 by using multiple sectorized antennas to further increase capacity. Typically three sectors per cell are used. (Both conventionally and in this document, the term “sector” applies even when there is only one sector in a cell). In each cell, a base station serves one or more sectors and communicates with multiple mobile stations 28, 30 in the cell. The communication between the base stations 22, 24, 26 and the mobile stations uses analog modulation (such as analog voice) or digital modulation (such as digital voice or digital packet data) to transmit and receive such analog or digital information. The forward link or downlink refers to the direction of communication from the base station to the mobile station. The other direction, i.e., the direction of communication from the mobile to the base station, is called the reverse link or the uplink.
A certain amount of bandwidth (spectrum) is used for such communication between the base station and the mobile station. Two separate spectrums can be allocated for the forward and reverse links as in the frequency division duplexing (FDD) scheme or one spectrum can be multiplexed in time to carry traffic in both directions as in a time division duplexing (TDD) scheme. The minimum unit of bandwidth needed in a cellular wireless system can be referred to as a carrier. As the amount of data traffic is increased, the number of carriers can be increased to provide more capacity. A carrier in a sector can handle up to a certain amount of data traffic, which is referred to as the capacity per carrier per sector or simply capacity. In general, the capacity is different in the forward and in the reverse links.
A wireless communication link between a transmitter and a receiver can be categorized as line-of-sight (LOS) or non-line-of-sight (NLOS). LOS refers to the case when the receiving antenna sees the transmitter directly, i.e., there is a direct path between the two. In NLOS environment, the transmitting and the receiving antennas do not see each other directly, but have multiple different paths over which radio waves can travel due to reflection from objects such as buildings and trees. In a perfect LOS environment, as in free space, there will be no fading. As the number of objects obstructing a direct path between the transmitter and receiver (called “scatterers”) increases, fading increases due to interference from multiple paths.
When a transmitter transmits a signal such as a sinusoidal waveform, the resulting received signal is the sum of multiple copies of the same signal. However, each copy can have different amplitude and phase depending on the characteristic of the corresponding path, where amplitude typically decreases as the length of the path increases and the phase increases linearly in path length. In addition, different materials have different reflection and absorption characteristics that effect amplitude and phase differently. Thus, these multiple paths can create multiple copies which constructively and/or destructively interfere with each other at a receiver.
If there are many scatterers that reflect waves, the resulting signal fading can be modeled as Rayleigh fading where the envelope of the received signal follows a complex Gaussian distribution when the transmitted signal is a pure sinusoid. This technique can be useful in modeling a scattering environment in which there are multiple paths without any dominating path because the sum of independent, identically distributed channel gains tend to have a Gaussian-like distribution as there are many paths. This is a good model of many NLOS environment. However, often real-world RF channels have a mixture of LOS and NLOS components. In this case, fading can be modeled as Rician or Nakagami fading. Rician fading models a mixture of LOS and Rayleigh fading paths. Nakagami fading can model fading that is more severe or less severe than Rayleigh fading.
Multi-path fading can also distort the signal if the delay spread is not negligibly small compared to the symbol duration since the fading becomes frequency selective in such an environment. If the delay spread is negligibly small compared to the symbol duration, the fading becomes flat in frequency and is called the flat fading. Various known equalization techniques can be used to remove inter-symbol interference caused by the frequency-selective fading.
Fading can change over time when the transmitter, receiver, or some surrounding objects move. Since the phase of each path can change 360 degrees when the path length changes by the wavelength, the fading can change in a very small scale. For example, fading can change from constructive to destructive when the receiver moves merely a fraction of the wavelength. In 2GHz Personal Communications Service (PCS) band, for example, this corresponds to only 1 or 2 inches. Therefore, in a mobile communication system, the mobility of the terminal can cause the fading characteristics to change quickly. For example, a mobile terminal moving at a pedestrian speed in the PCS band will see the channel fading characteristics change at a rate of a few times a second. However, if the mobile terminal is in a moving vehicle, the channel fading characteristics can change as often as a few hundred times a second. Even when there is no mobility of a terminal, fading can change over time if there are moving objects in the paths. In a mobile communication system, the main cause of time-varying fading is often due to movement of the terminal. For example, if a mobile terminal stops moving during signal transmission, the fading characteristics of the channel can stay constant for a long time. Accordingly, a user will likely experience bad reception if the user's mobile terminal stops moving when it is in deep fade.
In a cellular communication system, a downlink is a broadcast channel whereas the uplink is a multiple access channel from a single sector's perspective since a sector needs to be able to handle multiple mobile terminals. Therefore, to carry user data in the downlink, multiplexing is used. There are many forms of multiplexing, e.g., time-division multiplexing (TDM), frequency-division multiplexing (FDM), orthogonal frequency-multiplexing (OFDM) and code-division multiplexing (CDM). CDM, FDM, and static TDM (i.e., TDM where channel allocation does not change during the entire duration of a call) are typically used for low and fixed rate communication including voice.
Recently many efforts have been made in standardizing high speed data communication in cellular environment, which includes CDMA2000 and wide band CDMA (WCDMA). For example, 1× Evolution Data Only protocol (1×EVDO) described in “CDMA2000 High Rate Packet Data Air Interface Specification,” 3GPP2 C.S0024, which is referred to in this document as “3GPP2” and is fully incorporated herein by reference, supports data rates up to 3.1 Mbps in the downlink and 1.8 Mbps in the uplink. When the data rate becomes higher and higher, it becomes more important to use an efficient multiplexing scheme. An often good choice for such high speed applications is TDM because it can maximize burst throughput for each user and thus minimize latency.
Furthermore, when TDM is used, a smart scheduler in the downlink can take advantage of the time varying channel conditions to give higher scheduling priority to users whose channel condition has temporarily improved. The resulting gain from using such a smart scheduler is referred to as a “multi user diversity” gain since the gain becomes higher as there are more users. Multi user diversity gain is described in more detail in “Information capacity and power control in single-cell multi-user communications”, R. Knopp and P.A. Humblet, Proceedings of International Conference on Communications (ICC), 1995, Seattle, Wash., pp. 331-335, June 1995.
There are additional benefits in using TDM for downlink. One benefit is that it simplifies resource allocation since the resource is only one dimensional, i.e., time slots. In other multiplexing schemes, resource management can be a two-dimensional problem since code or frequency space also needs to be shared among users. This adds more complexity and tends to be less efficient than TDM. Another benefit of TDM for downlink is that it allows for dynamic scheduling of different types of contents to different users. For example, it can support mixture of unicast and broadcast services, where a unicast packet is received by a single user while a broadcast packet can be received by multiple users simultaneously.
Broadcasting and multicasting (BCMC) have been recently standardized in 3GPP2 and an enhanced version is being standardized in “Enhanced Broadcast and Multicast,” 3GPP2, QUALCOMM™. BCMC allows transmitting the same data from one or more sectors to mobiles. This is useful in sending, for example, TV or radio-like programs to a large number of mobile terminals in a large geographic area. In BCMC, no feedback channel is available on the channel condition. Therefore, adaptive modulation is not possible and the modulation needs to be done for the user who has the worst channel condition. Thus, broadcast typically does not perform as well as unicast in terms of total throughput. However, it is possible to improve the broadcast performance by taking advantage of the fact that multiple sectors can transmit the same information. Unlike in the unicast where each sector transmitter transmits different signals thus creating inter-sector interference to all neighbor sectors, transmitting the same signal from multiple sectors does not necessarily create such inter-user interference. However, the signals from multiple sectors are combined at the receiving antenna, thus creating self-interference. This interference can be either constructive or destructive depending on the phase of the signals.